This invention relates to a chair tilt lock mechanism having a single lever to lock the tilt mechanism where the lever is externally biased to prevent the mechanism from unlocking.
Chair tilt and height adjustment mechanisms having a single control lever or locking shaft to lock the backrest of the chair into the upright position and to adjust the height of the chair are known in the prior art. Typically, the locking shaft is carried by a housing so as to permit both rotation and lateral movement of the locking shaft relative to the housing, where the height adjustment of the chair seat is controlled by rotational actuation of a gas cylinder mounted to the housing.
A typical prior art tilt mechanism is disclosed in U.S. Pat. No. 5,427,434. The tilt lock mechanism utilizes a tilt lock pawl that is in fixed relationship with the locking shaft such that lateral movement of the shaft shifts the tilt lock pawl into a position whereby relative movement between the housing and bracket plate attached to the seat of the chair is precluded. The tilt lock pawl may be biased by a spring that also biases the locking shaft. When the locking shaft is laterally displaced a sufficient distance, the spring pivots in order to bias the locking lever against opposite lateral movement. The lateral bias prevents the locking shaft from returning to an unlocked position which would result in unwanted rotation of the chair from the upright position. Repeated lateral movement of the locking shaft, however, fatigues the spring and thus weakens the bias force exerted by the spring acting to prevent lateral movement of the locking shaft. Thus, the weakened spring and consequent inability of the bias force to prevent lateral movement of the locking shaft may result in an inadvertent release of the lock of the chair tilt mechanism that permits the chair to rotate.
It would be a desirable feature of a chair tilt mechanism to prevent inadvertent release of the tilt lock. This may be accomplished by providing a locking shaft that may be extended through both the housing and the bracket plate which is attached to the seat of the chair such that the bracket plate and housing are locked together and the locking shaft is compressively retained in the lock position by a barrier which precludes lateral movement of the locking shaft after the chair is tilted and locked in position.
There is, therefore, provided according to the present invention, a chair tilt locking mechanism that biases a locking shaft so as to prevent lateral movement of the shaft where the bias force results from an extension of a torsion spring compressively engaged against a barrier carried by the locking shaft.
The present invention is directed to an improved chair tilt lock mechanism that consists of a spindle housing having a horizontal axis and a bracket member that is pivotally mounted to the housing. A pivot pin extends laterally through both the spindle housing and the bracket member to permit relative rotational movement of the bracket member with respect to the spindle housing. The bracket member has a base that is so constructed and designed to permit the bracket member to be fastened to the under side of a chair. To raise or lower the chair seat, the spindle housing telescopically engages a spindle member where the spindle member may be raised or lowered by a piston arrangement. To move the chair from one location to another, the spindle member is supported by a plurality of legs attached to casters to permit rolling action of the chair. A locking shaft is carried by both the spindle housing and the bracket member and is so adapted for carriage to permit rotation and lateral movement of the locking shaft relative to both the spindle housing and the bracket member. A pawl plate is fixed to and integral with the locking shaft such that the pawl plate is carried in fixed relationship with the locking shaft during lateral or rotational displacement of the locking shaft.
The bracket member has a pair of rails extending from the base through which the pivot pin extends laterally to permit relative rotation between the spindle housing and the bracket member; one of the bracket member rails contains a slot and the other rail contains a pair of apertures. When the locking shaft extends laterally through an aperture, the spindle housing and bracket are locked and precluded from rotation relative to each other. If the locking shaft does not extend through either of the apertures, the chair may be tilted by rotation of the bracket member relative to the spindle housing. Thus, upon lateral extension of the locking shaft into either of the apertures, the bracket member will be locked rotationally to the spindle housing to preclude relative rotation between the two. To prevent the locking shaft from inadvertently retracting from an aperture to unlock the bracket member and thereby permit rotation or pivoting with respect to the spindle housing, a pawl member is mounted so as to rotate relative to the pivot pin where the pawl member has a first end that is biased by a first extension of a torsion spring carried by the pivot pin. The torsion spring has a second extension that is biased against the pawl plate in a compressive relationship throughout any rotational or lateral movement of the pawl plate. The pawl plate has a barrier rib that extends from its surface that prevents lateral movement of the locking shaft; lateral movement is precluded when the second extension of the torsion spring compressively bears against the barrier rib which occurs when the locking shaft extends through either aperture. During either rotational or lateral movement of the locking shaft, the second extension of the torsion spring remains in compressive engagement with the pawl plate. Thus, when the locking shaft extends through an aperture thereby locking the spindle housing and bracket member, the second extension of the torsion member laterally bears against the barrier rib of the pawl plate which prevents lateral movement of the locking shaft; sufficient lateral movement of the locking shaft would inadvertently unlock the spindle housing and the bracket member allowing relative rotation between the spindle housing and the bracket member.
The bracket member and spindle housing are biased by a coil spring such that the seat of the chair upon being tilted from the horizontal will be urged by the coil spring to return to the horizontal. The coil spring is carried by the spindle housing and associated with a bolt carried by the bracket member such that rotation of the bracket member relative to the housing will compress the coil spring; the compressed coil spring applies a restoring force to the bracket member thereby biasing the bracket member towards the horizontal. The improved tilt lock mechanism of this invention utilizes the second extension of a torsion spring to compressively engage a barrier rib that is carried by the locking shaft so as to preclude lateral movement of the locking shaft and thus prevent the bracket member from becoming unlocked rotationally with respect to the spindle housing.